The present invention relates generally to the field of imaging, and more specifically to the field of tomosynthesis. In particular, the invention relates to tomosynthesis systems and methods employing new scanning trajectories for an x-ray source and a detector to yield an improved image of an object.
Tomography is well known for both industrial and medical applications. Conventional tomography is based on a relative motion of the x-ray source, the detector and the object. Typically, the x-ray source and the detector are either moved synchronously on circles or are simply translated in opposite directions. Due to that correlated motion, the location of the projected images of points within the object moves also. Only points from a particular slice, typically called a focal slice, will be projected always at the same location onto the detector and therefore imaged sharply. Object structures above and below the focal slice will be permanently projected at different locations. Because of that, they are not imaged sharply and will be superimposed as a background intensity to the focal slice. This principle of creating a 3D image with one slice in focus (focal slice) using a discrete number of projections is called tomosynthesis.
Tomosynthesis systems for medical applications, typically use an x-ray source for producing a fan or cone-shaped x-ray beam that is collimated and passes through the patient to then be detected by a set of detector elements. The detector elements produce a signal based on the attenuation of the x-ray beams. The signals may be processed to produce a radiographic projection. The source, the patient, or the detector are then moved relative to one another for the next exposure, typically by moving the x-ray source, so that each projection is acquired at a different angle.
By using reconstruction techniques, such as filtered backprojection, the set of acquired projections may then be reconstructed to produce diagnostically useful three dimensional images. Because the three dimensional information is obtained digitally during tomosynthesis, the image can be reconstructed in whatever viewing plane the operator selects. Typically, a set of slices representative of some volume of interest of the imaged object is reconstructed, where each slice is a reconstructed image representative of structures in a plane that is parallel to the detector plane, and each slice corresponds to a different distance of the plane from the detector plane.
In addition, because tomosynthesis reconstructs three dimensional data from projections, it provides a fast and cost-effective technique for removing superimposed anatomic structures and for enhancing contrast in in-focus planes as compared to the use of a single x-ray radiograph. Further, because the tomosynthesis data consists of relatively few projection radiographs that are acquired quickly, often in a single sweep of the x-ray source over the patient, the total x-ray dose received by the patient is comparable to the dose of a single conventional x-ray exposure and is typically significantly less than the dose received from a computed tomography (CT) examination. In addition, the resolution of the detector employed in tomosynthesis is typically greater than the resolution of detectors used in CT examinations. These qualities make tomosynthesis useful for such radiological tasks as detecting pulmonary nodules or other difficult to image pathologies.
Though tomosynthesis provides these considerable benefits, the techniques associated with tomosynthesis also have disadvantages.
Reconstructed data sets in tomosynthesis often exhibit a blurring of structures in the direction of the projections that were used to acquire the tomosynthesis data. This is expressed in a poor depth resolution of the 3D reconstruction or depth blurring. The degree of depth blurring depends on the scanning parameters, the distance of the object from the plane of interest, and on the size and orientation of the object relative to the scan paths. For example, the traditionally used linear scanning trajectory may lead to a limited z-resolution, and a contrast that may depend on the orientation of the anatomy to be imaged, while the circular trajectory may lead to circular artifacts, caused by out-of-plane structures, that may be mistaken for pathology. The blurring of structures may create undesirable image artifacts and inhibit the separation of structures located at different heights in the reconstruction of the imaged volume. Some existing tomosynthesis reconstruction algorithms address streaking artifacts due to acquisition at discrete focal spot locations, but do not address depth blurring.
Also generally, the solid angular range and complexity of the acquisition projection geometries trade off against the physical limitations of an exam. For example, projection geometries which result from simple linear focal spot trajectories using a flat fixed detector over a small angular range are faster and less demanding of the focal spot positioning apparatus. The smaller exam time mitigates patient motion artifacts for medical imaging applications. However, because the angular range of these projection geometries is small, depth blurring will be more severe in volume reconstructions of objects imaged using such an acquisition configuration. Projection geometries which result from more complex three dimensional focal spot trajectories over a larger solid angular range where a custom geometry multiple surface detector may be repositioned during the scan require a longer exam time, demand more of the focal spot positioning apparatus, demand more of the detector positioning apparatus, and may require additional design considerations for the detector shape. The longer exam time exacerbates patient motion artifacts for medical imaging applications. However, because the solid angular range of these projection geometries is larger, the depth blurring artifacts in the reconstructed volume will be reduced relative to the simpler scan.
Therefore there exists a need to adapt the current tomosynthesis systems to provide for new scanning trajectories to address the depth blurring of the imaged object by using more general projection geometries that may be more suited to reconstructing the region of interest and the anatomy to be imaged.